Wireless regional area networks (WRANs) operate using a cognitive radio-based approach in which the target spectrum includes unused channels that have been allocated for television broadcast services. In order to avoid interference, TV broadcast stations that are being used in any given region may be detected and avoided by devices functioning as part of a WRAN. However, some low-powered devices, such as wireless microphones and other devices licensed under Part 74 of the Federal Communication Commission rules (i.e., Part 74 devices), are more difficult to detect and avoid than TV broadcast stations because of their low transmit power and other factors.
For example, some wireless microphones and other Part 74 devices have a limited coverage of around 200 meters. Thus, systems located far away (e.g., a base station located 30 km away) are unable to sense and protect those low-power devices. One proposed solution to this problem involves the use of a beacon device associated with a low-power device. The beacon device has a much larger coverage (e.g., around 35 km) and is thus able to alert other wireless systems to the presence of the low-power device. The challenges of designing such a beacon device include cost and high reliability of beacon signal detection.
In one proposed design, a long symbol is used for coping with multipath fading without using complicated signal detection methods, such as equalization, channel estimation, or OFDM modulation. One of the disadvantages of this design is that a long symbol implies a low data rate, which in turn requires a long sensing period for detecting the beacon signal. For example, about 4.567 msec are needed for detecting a 24-bit burst, and about 68.5 msec are needed for detecting a 360-bit beacon PSDU. Since a long series of synchronization bits is sent before sending a beacon PSDU, this beacon design fails to meet the requirement in 802.22 functional requirements document that the transmission of a low-power device needs to be detected and protected within two seconds. Therefore, there is a need in the art for an improved method for providing a beacon signal to protect a low-power device in a wireless network.